The invention relates to an apparatus for switching between an electrical drive and a manual drive mode of an optical instrument, and more particularly, to an optical instrument such as stereo-microscope which is normally electrically driven but which is also provided with a switching device to permit a manual drive.
An optical instrument, for example, a stereo-microscope, is known which may be electrically driven to change its magnification, and finds applications in surgical operations. A conventional electrically driven zoom stereo-microscope is not provided with a manual drive mechanism, and hence a change of the magnification is inhibited in the event of occurrence of a failure in the electrical drive such as motor. In particular, if the failure of the motor occurs during a surgical operation for which the microscope is used, there has been no accommodation for emergency, resulting in a hazardous risk. Frequently a resort must be had to an expert for the repair of the electrical drive, and it takes a relatively long period of time until the microscope is reusable. In the meantime, there can be made no use of the microscope.